1. Field of the Invention
The present invention relates to a method and an apparatus for inspecting a dust particle and, more particularly, to a method and an apparatus adapted for inspection of a dust particle on a reticle or the like.
2. Description of the Related Art
In a semiconductor exposer, a reticle dust inspection apparatus is employed to detect a dust particle 5 deposited on the surface of a reticle 1. Such a reticle dust inspection apparatus is used normally in combination with a reticle changer. When the reticle 1 is displaced from a reticle cassette housing carrier in the reticle changer to a predetermined position in the semiconductor exposer or is returned therefrom, the reticle 1 is inspected by the reticle dust inspection apparatus.
An exemplary conventional reticle dust inspection apparatus known heretofore comprises a He-Ne or Ar laser beam source, a scanner for scanning the surface of an inspected article with a light beam emitted from the laser beam source, an optical detector for detecting the light beam reflected and diffracted on the surface of the inspected article, and a stage for setting the inspected article thereon and displacing the inspected article in a predetermined direction. Meanwhile the scanner consists of a polygon mirror and an f-.theta. lens for example, and the optical detector consists of a condenser lens and a photo sensor.
In inspection of a dust particle 5 on the surface of the reticle 1, first the reticle 1 is set on the stage. Then the stage is displaced by a driving mechanism in a predetermined direction (e.g., Y-direction), and simultaneously therewith the surface of the reticle 1 is irradiated, at an oblique incidence angle, with the light beam emitted from the He-Ne or Ar laser beam source, whereby the reticle surface is scanned at a certain angle formed with respect to the Y-direction. As typically illustrated in FIG. 1A, the laser beam is reflected from the surface of the reticle 1 in case none of dust particle is existent on the reticle surface. However, when a dust particle 5 or the like is existent on the reticle surface as typically illustrated in FIG. 1B, the laser beam is diffracted with diffused reflection by such dust particle 5. In this case, a portion of the diffracted beam 4 is selectively received by the optical detector and then is converted into an electric signal to detect the dust particle.
Diffraction of the laser beam 2 is caused also by a pattern formed on the reticle 1. The diffracted beam 4 resulting from the dust particle is projected omnidirectionally at predetermined diffraction angles. Meanwhile the diffracted beam 4 resulting from the pattern has such attribute that it is projected in a direction of 90.degree. with respect to the pattern. The directions of the pattern formed on the reticle are generally at angles of 0.degree., 45.degree. and 90.degree. with the Y-direction. As illustrated in a typical plan view of FIG. 1C, the pattern and the dust particle 5 are strictly discriminated from each other by first irradiating the laser beam 2 to the reticle 1 in such a manner that the direction of the irradiated laser beam 2 to the reticle has a fixed angle .alpha. of, e.g. 75.degree., with the Y-direction, and then receiving merely the diffracted beam 4 at such angle .alpha. by the optical detector.
In the conventional reticle dust inspection apparatus, it is generally customary to employ a He-Ne or Ar laser beam source. Since such laser beam source has a low coherence with its coherence distance being approximately 0.1 m, so that the light intensity of the diffracted beam is low and the signal-to-noise ratio is inferior. Consequently there arises a problem that a high precision is not achievable in detection of a dust particle or the like. The minimum detectable size thereof is 0.7 .mu.m or so. The light intensity of the diffracted beam can be increased by the use of a laser beam source having a longer coherence distance and a higher coherence, and therefore it is possible to enhance the precision in detecting a dust particle or the like. However, in the use of a laser beam source of a high coherence, another problem arises that a speckle pattern is generated due to corpuscles of ultrafine dust or the like floating in the air in an optical path proximate to the reticle, hence failing in proper detection of the dust particle existent on the reticle surface.